Project Summary/Abstract When a T cell senses an infection, it undergoes explosive proliferation. However, this proliferation is transient and does not continue excessively to become cancer. How is such exquisite control of cell division possible? Although we are familiar with the genes that are involved in this process, we are still relatively uninformed as to how these genes are combined into circuits that enable precise temporal control. This question of population size control also has practical relevance for improving adoptive cell transfer immunotherapy for cancer, an increasingly successful technique whereby an individual's T cells are reengineered to target cancer. Existing engineered T cells do not furnish clinicians with the ability to carefully regulate their numbers?similar to a drug without the ability to control dosage. Consequently, side effects associated with either insufficient or over-proliferation can occur. To enhance our understanding of T cell population expansion, I propose to take an engineering approach in which new genetic circuits will be synthetically constructed in T cells that allow population size to be controlled artificially and dynamically. Proteins that promote either cell division or death will synthesized by the cells in response to chemical stimuli, and these growth and death components will be combined in different ways to create different user-adjustable behaviors in cell populations. Using these systems, I plan to determine the relationship between genetic circuit design and the resulting dynamics of population expansion and contraction. The proposed research will provide a generalizable characterization of the types of regulatory elements that can produce different population size dynamics. Some of these synthetic circuits themselves may prove useful tools in the development of new engineered T cell therapies for cancer. Furthermore, the relationships uncovered here may shed light on the design principles governing the wiring of native proliferative control systems.